1. Field of the Invention
The present invention is directed generally to determining the tension on a replacement device and more particularly to the tension on, and hence the optimal placement of, a cruciate ligament replacement.
2. Description of the Prior Art
It is known to replace various parts of the human body with artificial parts. In one application, ligaments which have been damaged beyond repair or diseased can be replaced with artificial ligaments. The artificial ligament should function the same as, or as closely as possible to, the original ligament. To achieve proper functioning of the artificial ligament, the position of the ligament must be precisely determined so that the tension experienced by the ligament as the articulated bones to which it is attached move relative to each other approximates that experienced by the original ligament. However, due to the unavoidable differences in joint configurations (both geometric and kinematic) from one patient to the next in the normal knee, which is compounded in joints which have been previously operated upon, are diseased, or the like, the optimal placement of any device purporting to replicate the function of the original is not obvious without precise measurements of the nature of the forces to be experienced in situ.
U.S. Pat. No. 4,712,542 discloses a method and apparatus for establishing in situ ligament graft orientation and isometry, particularly for replacement of the anterior and posterior cruciate ligaments. The ligament graft is extended from one fixation site and is attached to a sled slidably carried by a frame. The frame is skeletally mounted to the other fixation site. A thumb nut and lead screw assembly carried by the frame are operative to compress a spring in the sled and move the sled in a direction effective to increase the tension in the graft. A pointer cooperates with force indicia to provide an indication of force while two means are provided for indicating the longitudinal position of the sled relative to the frame. Various measurements are taken with the articulated bones at various degrees of flexion and the ligament graft exposed to a constant tension. Isometry is achieved when the relative position of the frame and sled remain substantially unchanged through the entire range of motion.
The aforementioned device is somewhat cumbersome to use in that for each position of the articulated bones, the pointer must be read to insure that the force on the ligament graft is constant. If not, then the thumb nut must be operated to reestablish the desired tension. Thereafter, the position of the sled relative to the frame is determined. After several readings, if the position of the sled relative to the frame stays within acceptable limits, the fixation sites are deemed acceptable. Additionally, error may be introduced in the aforementioned device by virtue of the use of the spring in line with the ligament graft. The spring's expansion and contraction effectively changes the fixation site thus introducing errors in the tension measurements.
The need exists for an apparatus which is easy to use and provides an accurate reading of the tension experienced by the ligament with respect to proposed fixation sites. Preferably, a continuous reading of the tension is provided so that inferences needn't be drawn based on changes in length at a few points within the range of movement of the articulated bones.